It is proposed to continue our investigation of the biosynthesis of the antileukemic alkaloids of Cephalotaxus plants. There are three objectives to this research. The first is to elucidate the mode of biosynthesis of the parent alkaloid cephalotaxine from tyrosine. This will be accomplished by administration of specifically labeled forms of tyrosine to Cephalotaxus plants followed by appropriate degradations. The second objective is to determine the steps in the biosynthesis of deoxyharringtonic acid and isoharringtonic acid. This part of the investigation will also rely on the administration of specifically labeled precursors to Cephalotaxus plants with subsequent degradative work. The third objective is to gain some insight into the biochemical processes involved in the metabolism of cephalotaxine and deoxyharringtonine. This will be accomplished by feeding experiments and by short-term exposure of Cephalotaxus plants to a steady-state concentration of C14-carbon dioxide.